Leakage detection

ABSTRACT

A method of testing the leakage rates from tanks for containing volatile liquids such as petrol or diesel, with a view to determining whether the tank leaks excessively. The tank is at least partially filled during testing in order to save time. The leakage is detected by sealing the tank and measuring the drop in pressure in the tank over a period of time. In order to measure the rate of leakage, a first measurement is taken during which an amount of 380 ml is removed from the tank over a short period whilst otherwise sealed, with the resultant pressure drop being measured as a calibrated value. The tank is then re-sealed and the pressure drop over the period of an hour is compared with the calibrated value.

This invention relates to a leakage test which is particularly suitablefor the testing of underground storage tanks such as those found atpetrol stations and chemical works, and even milk storage tanks.

Because leakage from underground storage tanks cannot be readilydetected externally, internal .testing methods are commonly used. .Someyears ago the most common method of testing the integrity of undergroundstorage tanks was a high pressure test. This method includes therequired step of removing the contents of the tank, these normally beingvolatile and prone to spontaneous combustion under the influence ofelevated pressures, before supplying the tank with gas at high pressure.In order to detect leakage from the tank, the tank is otherwise sealedoff at the input and output connections, the pressure inside the tank iselevated to around 3,500 mb above atmospheric pressure and the pressureis read both initially and after some time finally by means of apressure gauge fluidly connected to the tank interior. The detection ofa pressure loss indicates the possibility of a leaking tank.

There are significant drawbacks to this high pressure testing method.The need to first empty the tank leads to a time consuming and costlytest which requires a second tank in which the normal contents can bestored during the test. Perhaps the greatest drawback is that ofinsensitivity. Most underground spirit-containing tanks have largeullage spaces, and only relatively small changes in gas pressure causedby normal rates of leakage during a test. Furthermore, the highpressures are monitored by mechanical pressure gauges, which are bynature insensitive to the pressure changes created by these normal ratesof leakage. It is thought that substantial leakages went unnoticed bythis testing method, which has resulted in the release of significantquantities of pollutant into environments surrounding the leaking tanks.Furthermore, the amount of energy stored in a tank at such an elevatedpressure is excessive and can be dangerous both to the tank itself andthe testing personnel.

New and stricter legislation has now been introduced in some countriesto protect the environment by placing lower limits on the amount ofleakage permissible from a store of liquid pollutant (defined by theauthorities as any liquid other than water), and these standards havemade the high pressure tests obsolete in those countries. According tocurrent legislation in Europe and the United States of America theintegrity of a pollutant storage tank will be certified only if its rateof leakage into the surrounding environment is less than of 380 mlduring the course of a single hour. Consequently, any testing methodswhich the authorities approve must be able to reliably detect a leakageoccuring at the specified rate.

Thus, and particularly in the case of the largest of tanks, only a smallnumber of testing methods have been approved by the authorities formeeting this sensitivity requirement.

According to one known test, an electronic sensor is lowered into theliquid contents of a tank, and the liquid level is directly measured.This test is however time-consuming and is generally undertakenovernight when the tank is out of use, and the long monitoring periodcan lead to inaccuracies due to effects such as thermal expansion of thetank and its contents. It is also inherently-unsuitable for tankscontaining liquids which react with the material of the sensor, such asacids.

In another known test, the tank is sealed and a vacuum is applied. Airentering the tank via a leak site causes a bubbling sound which can bepicked up by sonic monitoring equipment. This monitoring method howevercannot detect leaks in pipelines associated with the tanks which requireseparate testing.

All of the known tests suffer from deficiencies where the tank issurrounded by a water table. Depending on the relative levels of liquidinside and outside the tank, a leak can be exacerbated, completelymasked, or water can enter the tank via the leak site. With certainliquids, an influx of water can be detected as it settles at the base ofthe tank, but this is impossible for other liquids with which waterreadily mixes or forms an emulsion.

It is furthermore the case that the currently approved methods oftesting can be inhibitively expensive, as a combined result of thedown-time required and the actual cost of the equipment required for thetesting. Indeed in some cases businesses have become unviable and haveclosed due to the prohibitive cost of the presently approved methods.

Until now it has been believed that a test utilising a method of testingduring which the gas pressure is monitored would be unworkable underpresent regulatory conditions. Such testing by pressure monitoring hasin the part been discounted for a number of reasons. Due to theregulations against high pressure testing, it was thought thatinsufficient sensitivity could be obtained to obtain meaningful resultswithin an economic timescale, bearing in mind that a tank must be keptout of service during a test of its leakage rate. Furthermore, becausethe known pressure monitoring test requires the removal of the tank'scontents before testing, pressure monitoring was thought to be aninconvenient and inefficient method made obsolute by the new testingmethods.

The applicant has however surprisingly found that a pressure monitoringtest is workable under present regulations, and indeed a number ofsignificant advantages accrue.

According to the present invention, there is provided a method oftesting for fluid leakage from an underground storage tank normallycontaining a liquid pollutant, the test comprising the steps of fluidlysealing the tank, fluidly connecting a pressure sensitive means to theinterior of the tank, and monitoring the pressure sensed over a periodof time, characterised in that the monitoring is carried out whilst thetank is partially filled with its normal liquid contents and alsopartially filled with a gas, the effect of allowing the normal liquidcontents to remain being to amplify pressure variations caused byleakage from the tank.

The reservoir is preferably only partially filled with the volatileliquid for the duration of the test, the remaining ullage space beingfilled with a gas. This is the condition in which a reservoir is foundduring everyday use.

The test is particularly suitable for the testing of large tanks havingan ullage space above around 4,000 liters. Since the changes in pressureduring a test are approximately inversely proportional to the volume ofgas in the reservoir and any connecting conduits between the reservoirand the pressure sensitive means, it is preferred that the reservoirdoes not contain an excessive gas volume during testing. In order toachieve the desired sensitivity to test a leakage of 380 mls per hour,it is preferred that the test is carried out with an ullage space or gasvolume at or less than around 4,000 Liters in the reservoir.

The test is preferably carried out at a pressure of between 250 mb aboveatmospheric pressure and 250 mb below atmospheric pressure. This upperlimit affects the pressure above which it is generally considered safeto work without the risk of explosion or other damage to the tank and/ortesting personnel, although the applicant considers that testing ispossible at much higher pressures according to the present invention. Atpressures below the lower limit, there is a danger of causing thecollapse of an underground storage tank.

The choice of whether a reduced pressure, and elevated pressure, orindeed atmospheric pressure is used during the course of the test willdepend on the prevalent conditions, particularly the height of the waterlevel surrounding the tank compared to the level of liquid within thetank. It will be appreciated that, should these levels be similar, atest at atmospheric pressure will lead to little or no leakage occuringduring the course of a test even if the tank would release liquidpollutant when the water table is at a lower level compared to that ofits contents.

The method preferably includes the step of introducing a gas into thereservoir to elevate the pressure in the interior above atmosphericpressure. The use of an elevated pressure reduces the possibility of.erroneous test result due to a high water level surrounding the tank,which may tend to counteract the detection of a leak. The contents ofthe tank are forced from the site of a leak by the elevated internalpressure. This further facilitates the detection of the position of aleak site, since the emission of gas or fluid is detectable by means ofa leak detector spray at the suspected site of the leak.

If the pressure is elevated, it is however essential that the pressureinside the reservoir be regulated to ensure that it does not rise past asafe level and preferably the pressure does not rise above approximately200 mb above atmospheric pressure during monitoring.

The pressure sensitive means is preferably an electronic pressuretransducer, which can preferably sense changes in pressure of one tenthof a millibar.

The said elevated pressure is preferably at least 50 mb aboveatmospheric pressure.

An elevated pressure of above in the region of 150 mb is preferred sincethis will generally ensure that the pressure inside the partially-filledtank (which may have a liquid level between 1 m and 3 m below groundlevel) exceeds pressure externally of the tank, irrespective of theprevailing height of the water table surrounding the tank, and a readilydetectable leakage rate will be obtained if the tank leaks excessivelyin normal use.

The pressurised gas may be supplied by means of a high pressure gassupply, in which case a low pressure regulator is preferably provided toprevent the possible supply of excessive pressure within the tank. Theregulator preferably embodies a pre-set pressure-relief valve, and thepressure at which this relief valve is actuated may be in the region of30 mb above that of the said elevated pressure.

The gas supplied is preferably an inert and/or non-combustible gas suchas oxygen-free nitrogen, which serves to inhibit explosions in thereservior. Other gases which may be suitable include argon and helium,depending on their compatibility with the liquid contents of the tank.

Whilst allowing the contents of the tank to remain during the test, theachievement and maintenance of equilibrium within gas/liquid contentsmust be taken account of, since significant pressure variations may becaused by disturbance of the liquid particularly it it is a volatilespirit such as petrol. Therefore in carrying out the testing, effortsare preferably made to eliminate possible causes of fluid flow or otherdisturbances in the body of liquid, for if disturbances do occur, it maybe necessary to allow time for the reservoir to once again reachequilibrium.

Where the reservoir is divided into two or more gas-filled compartmentsfluidly interconnected by the liquid contents, gas is preferablyintroduced into each compartment, or removed therefrom if the pressureis to be reduced, simultaneously to equalise pressures without causingfluid flow between the compartments.

Furthermore, where the reservoir comprises a main tank and a number ofpipelines, it is preferred that the pipelines are sealed at least somedistance from the main tank. By this method, an entire reservoirincluding its network of supply pipelines can be tested for leaks byallowing the elevated fluid pressure to permeate the system to theextent to which testing is required.

The reservoir may be a fuel tank at a petrol filling station comprisinga tank body, a fill pipe, a vent and a suction line leading to the fueldispenser pumps. It is to be noted that the prior and alternativemethods do not take into account of leakage from the the network ofsupply pipelines connected to the main reservoir tank. However thesuction line is in use fuel-filled and therefore according to thepresent invention may simply be blanked off by means of a spade plate,preferably close to the pump site so that the test includes the suctionline itself.

The gas supply means may be sealingly connected to the opening of thefill pipe, and the vent may be sealingly connected to the fill pipe bymeans of a pressure balance tube.

An embodiment of the present invention will now be described by way ofexample only with reference to the accompanying diagrams, wherein:

FIG. 1 is a schematic view of the a petrol tank at a filling stationbeing tested according to an embodiment of the present invention;

FIG. 2 is a schematic illustration of the tank connections made duringthe low-pressure test;

FIG. 3 is a schematic view of the test control unit utilised in thisembodiment; and

FIG. 4 is a graph illustrating pressure monitored during a testaccording to the present invention.

Referring to the diagrams and firstly to FIG. 1, an underground petroltank is shown to comprise a tank body 2, a suction line 4 leading fromthe base of the tank body to a pump dispenser 10, a fill pipe 6 and avent pipe 8, which are standard features of filling station tanks toallow the provision of a continuous supply from the pump dispenser 10 bythe regular filling of the tank 2 by means of the fill pipe, the ventbeing opened to prevent the build-up of excessive pressure within thetank during filling.

In the leakage test of the embodiment shown, the tank is filled almostfully to the level 12 shown to leave a small ullage 14 of up to 4000Liters of gas-filled space in the tube. The tank must be isolated fromthe surrounding environment, and thus the supply lines must be blockedoff. The suction line 4 is in use filled with the liquid product, andmay simply be blanked off at the end 16 nearest the pump dispenser 10.The fill pipe 4 and vent 8 are capped off by means of a fill pipe cap 18and a vent pipe cap 20 shown in greater detail in FIG. 2. These caps 18and 20 are connected via a pressure balance tube 26, and the fill pipecap 18 is connected to a control unit which in turn connects with anitrogen, oxygen free (NOF) gas supply cylinder by means of ahigh-pressure nitrogen supply tube 27. The control unit connects withthe fill pipe cap by means of two pipelines, a nitrogen supply line 29,and a pressure sensing line 28.

The control unit which forms a further aspect of the present inventionis shown in greater detail in FIG. 3, and comprises various pipelineconnections 40, inputs and outputs, multi-outlet precision controlvalves 42, an sensitive electronic pressure transducer 30, a pressureguage 32, a low pressure regulator 34, a vacuum pump 36 and a barometer38. The transducer 30 sensor changes in pressure at incements of onetenth of a millibar.

In effecting the test once the connections shown in FIG. 1 are made, NOFis supplied to the control unit 22 by opening the valve of the gascylinder 24, and this input pressure is monitored by the mechanicalpressure gauge 32. The NOF is supplied to the tank 18 via the lowpressure regulator 34 which ensures that the NOF is supplied at a lowpressure below that at which the regulator is set, which in thisembodiment is 220 mb, the output 44 and the supply pipe 29. The NOFsupplied enters not only the fill pipe 6, but also the ullage 14 via thebalance line 26 and vent pipe 8.

It is to be mentioned that, where the contents of the tank are volatileit may be preferably to apply a vacuum to the tank before supplying NOFso as to extract much of the initial oxygen content of the tank beforethe pressure is elevated.

The overall fluid pressure in the tank 2 is increased to approximately190 mb by the gradual supply of NOF from the cylinder 24, at which timethe supply lines are sealed and monitoring may begin.

This monitoring is achieved via the pressure return line 28, connectedto an input 46 or 48 which each lead to the electronic pressuretransducer 30. The transducer was chosen particularly for its safetycharacteristics, as it is imperative that no electrical sparking orother combustive elements are caused by the operation of the transducer.The pressure transducer used is manufactured by A Drug, a Britishcompany, based in Leicestershire under the model reference DPI 700 IS.

Once the connecting valves between the tank 2 and the transducer 30 areopened, the pressure is first allowed to stabilise. Once equilibrium isreached, perhaps at 180 mb, a subsequent pressure drop will most likelyindicate leakage from the tank. Other considerations such as atmosphericpressure and temperature changes during the course of the test can be toblame, however these can also be monitored by means of the barometer 38and a thermometer, and the test can be repeated if it is suspected thatthese changes are responsible for the pressure variation within thetank.

A graph illustrating the pressure variation with time during testing isshown in FIG. 4. Curve 100 shows the results of monitoring for a tankwhich does not leak, whereas curve 102 illustrates the results obtainedduring testing of a leaking tank.

The highest pressure is attained at point 104 on the curves, at whichpoint the supply of NOF is cut off. The pressures then vary considerablyover the subsequent fifteen minute period, at which point a saturatedvapour pressure is attained and the pressure in a fully sealed tankstabilises, as illustrated by curve 100. In the leaking tank, however,the pressure gradually decreases after the attainment of a saturatedvapour pressure, and indeed curve 102 shows that the pressure in aleaking tank, although initially at the same elevated pressure as thefully sealed tank, will always be less than that for the sealed tank, aswould be expected.

One problem experienced with other types of testing is the detection ofthe point at which the 380 ml/hour rate of leakage is exceeded, sincefor example the electronic liquid level detector must take account ofthe surface area of liquid in the tank. With the method of the presentinvention however, a 380 ml amount of liquid is first removed by meansof a calibration valve and a submerged tube whilst the tank pressure ismonitored at the elevated pressure. Thereby an accurate and calibratedvalue is obtained for the pressure drop limit which takes account of allprevailing atmospheric conditions. The pressure drop shown on thetransducer is then used to provide an upper limit of pressure dropallowed during a one-hour test. If this pressure drop limit is exceeded,an unacceptable leakage rate from the tank has been found.Alternatively, this limit may be calculated theoretically based on thevolume of gas within the tank and other variables, however thistheoretical approach is likely to be more time-consuming and lessaccurate.

Once an unacceptably large leak has been found, it may be located bymeans of for example a leak revealing coating spray, and the vacuum pump36 may be operated to prevent further pollution of the surroundingenvironment by holding the spirit product in the tank pending thearrival of a tanker vehicle to remove the product from the tank and therepair of the leak.

By the use of two or more pressure return lines and connections 46, 48from different tanks, a plurality of tanks may be tested simultaneously.

It will be appreciated that the present invention is not limited to thepressure and ullage values mentioned in the above embodiment, and that arange of these values is possible according to the invention, dependingupon the limits of accuracy and safety required and furthermore it isthe case that an equivalent procedure may also be effected using amedium vacuum pressure inside the tank if desired.

I claim:
 1. A method for the testing for fluid leakage from a storagetank normally containing a liquid pollutant, the test comprising thesteps of fluidly sealing the tank, fluidly connecting a pressuresensitive means to the interior of the tank, and monitoring the pressuresensed over a period of time, the monitoring being carried out whilstthe tank is partially filled with its normal liquid contents and alsopartially filled with a gas, the tank comprising two gas-filledcompartments located above and fluidly interconnected by the liquidcontents, and wherein the test comprises one of admitting gas directlyto, and extracting gas directly from, each of said compartments.
 2. Amethod according to claim 1 wherein the test is carried out at apressure between 250 mb above atmospheric pressure and 250 mb belowatmospheric pressure.
 3. A method according to claim 1 including thestep of introducing a gas into the tank after sealing thereof to elevatethe pressure in the interior.
 4. A method according to claim 3 whereinpressurised gas is supplied by means of a high pressure gas supply, anda low pressure regulator is used to prevent the possible supply ofexcessive pressure to the tank.
 5. A method according to claim 3 whereinthe gas is an inert non-combustible gas.
 6. A method according to claim5, wherein the gas is one selected from the group of oxygen-freenitrogen, argon and helium.
 7. A method according to claim 1 wherein gasis introduced into each compartment, or removed therefrom if thepressure is to be reduced, simultaneously to equalise pressures withoutcausing fluid flow between the compartments.
 8. A method according toclaim 1 for testing a fuel tank at a petrol filling station comprising atank body, a fill pipe, a vent and a suction line leading to fueldispenser pumps, wherein the said two gas-filled compartments are a) aspace above the main body of the liquid contents, and b) a space abovethe liquid contents within the fill pipe.
 9. A method according to claim8 wherein the suction line is blanked off close to the pump site so thatthe test includes the suction line itself.
 10. A method according toclaim 8 wherein a gas supply means is sealingly connected to the openingof the fill pipe.
 11. A method according to claim 8 wherein the vent issealingly connected to the fill pipe by means of a pressure balancetube.
 12. A method for the testing for fluid leakage from a storage tanknormally containing a liquid pollutant and having a fill pipe and avent, the test comprising the steps of fluidly sealing the tank, fluidlyconnecting a pressure sensitive means to the interior of the tank, andmonitoring the pressure sensed over a period of time, the monitoringbeing carried out whilst the tank is partially filled with its normalliquid contents leaving a head space which is filled with a gas, andwherein the test further comprises balancing the pressure in the headspace and a space above the liquid contents within the fill pipe duringa pressure change in the tank.
 13. A method according to claim 12wherein the test is carried out at a pressure between 250 mb aboveatmospheric pressure and 250 mb below atmospheric pressure.
 14. A methodaccording to claim 12 including the step of introducing a gas into thetank after sealing thereof to elevate the pressure in the interior. 15.A method according to claim 14 wherein pressurised gas is supplied bymeans of a high pressure gas supply, and a low pressure regulator isused to prevent the possible supply of excessive pressure to the tank.16. A method according to claim 14 wherein the gas is an inertnon-combustible gas.
 17. A method according to claim 16, wherein the gasis one selected from the group of oxygen-free nitrogen, argon andhelium.
 18. A method according to claim 12 wherein gas is introducedinto the head space and the space within the fill pipe, or removedtherefrom if the pressure is to be reduced, simultaneously to equalisethe pressures without causing fluid flow therebetween.
 19. A methodaccording to claim 12 wherein a gas supply means is sealingly connectedto an opening of the fill pipe.
 20. A method according to claim 12wherein the head space is sealingly connected to the fill pipe by meansof a pressure balance tube.